The present invention relates to a process for polymerizing vinylidene aromatic monomers, such as styrene, to produce polymers having a high degree of syndiotacticity while controlling molecular weight. Such polymers may be usefully employed in the preparation of solid objects and articles such as a moldings, films, sheets and foamed objects by molding, casting or the like process.
In U.S. Pat. No. 4,680,353 there is disclosed a process for the preparation of polymers of vinylidene aromatic monomers having a stereoregular structure of high syndiotacticity, by the use of certain coordination catalysts. Particularly disclosed were the reaction products of a titanium compound and an organoaluminum compound, especially polymethylaluminoxane.
In U.S. Pat. No. 5,066,741 there are disclosed certain cationic metal compounds formed by reacting a metal complex with ammonium or phosphonium salts of Bronsted acids containing a non-coordinating compatible anion or with cationic oxidizers containing a non-coordinating compatible anion. The complexes are usefully employed as catalysts in the polymerization of polymers of vinylidene aromatic monomers having a stereoregular structure of high syndiotacticity. In addition, this reference teaches that an aluminum alkyl compound may additionally be utilized with the catalysts in order to protect them from deactivation. For the teachings contained therein, the aforementioned U.S. Pat. Nos. 4,680,353 and 5,066,741 are herein incorporated in their entirety by reference thereto.
It is previously known to control molecular weight in a Ziegler-Natta catalyzed coordination polymerization of ethylene and other .alpha.-olefins by the use of certain materials known as chain transfer agents. Hydrogen is especially useful in this application. Molecular weight control is desirable to produce lower molecular weight polymers for specific end-uses. In the polymerization of vinylidene aromatic monomers, hydrogen has not proven to be an acceptable chain transfer agent. It would be desirable if an alternative technique for controlling molecular weight in the polymerization of vinylidene aromatic monomers to produce syndiotactic polymers were available.